Stress corrosion cracking behavior of buried oil and gas pipeline steel under the coexistence of magnetic field and sulfate-reducing bacteria

Magnetic field and microorganisms are important factors influencing the stress corrosion cracking(SCC)of buried oil and gas pipelines. Once SCC occurs in buried pipelines, it will cause serious hazards to the soil environment. The SCC behavior of X80 pipeline steel under the magnetic field and sulfate-reducing bacteria(SRB) environment was investigated by immersion tests, electrochemical tests, and slow strain rate tensile(SSRT) tests. The results showed that the corrosion and SCC sensitivity of X80 steel decreased with increasing the magnetic field strength in the sterile environment. The SCC sensitivity was higher in the biotic environment inoculated with SRB, but it also decreased with increasing magnetic field strength, which was due to the magnetic field reduces microbial activity and promotes the formation of dense film layer. This work provided theoretical guidance on the prevention of SCC in pipeline steel under magnetic field and SRB coexistence.

Corrosion and Electrochemical Behavior of 316L Stainless Steel in Sulfate-reducing and Iron-oxidizing Bacteria Solutions

Corrosion and electrochemical behavior of 316L stainless steel was investigated in the presence of aerobic iron-oxidizing bacteria (IOB) and anaerobic sulfate-reducing bacteria (SRB) isolated from cooling water systems in an oil refinery using electrochemical measurement, scanning electron microscopy (SEM) and energy dispersive atom X-ray analysis(EDAX). The results show the corrosion potential and pitting potential of 316L stainless steel decrease distinctly in the presence of bacteria, in comparison with those observed in sterile medium under the same exposure time. SEM morphologies have shown that 316L stainless steel reveals no signs of pitting attack in the sterile medium. However, micrometer-scale corrosion pits were observed on 316L stainless steel surface in the presence of bacteria. The presence of SRB leads to higher corrosion rates than IOB. The interactions between the stainless steel surface, abiotic corrosion products, and bacterial cells and their metabolic products increased the corrosion damage degree of the passive film and accelerated pitting propagation.

Molecular Characterization of Sulfate-Reducing Bacteria Community in Surface Sediments from the Adjacent Area of Changjiang Estuary

Sulfate-reducing bacteria(SRB),which obtain energy from dissimilatory sulfate reduction,play a vital role in the carbon and sulfur cycles.The dissimilatory sulfite reductase(Dsr),catalyzing the last step in the sulfate reduction pathway,has been found in all known SRB that have been tested so far.In this study,the diversity of SRB was investigated in the surface sediments from the adjacent area of Changjiang Estuary by PCR amplification,cloning and sequencing of the dissimilatory sulfite reductase beta subunit gene(dsr B).Based on dsr B clone libraries constructed in this study,diversified SRB were found,represented by 173 unique OTUs.Certain cloned sequences were associated with Desulfobacteraceae,Desulfobulbaceae,and a large fraction(60%) of novel sequences that have deeply branched groups in the dsr B tree,indicating that novel SRB inhabit the surface sediments.In addition,correlations of the SRB assemblages with environmental factors were analyzed by the linear model-based redundancy analysis(RDA).The result revealed that temperature,salinity and the content of TOC were most closely correlated with the SRB communities.More information on SRB community was obtained by applying the utility of Uni Frac to published dsr B gene sequences from this study and other 9 different kinds of marine environments.The results demonstrated that there were highly similar SRB genotypes in the marine and estuarine sediments,and that geographic positions and environmental factors influenced the SRB community distribution.

Analysis of cultivable aerobic bacterial community composition and screening for facultative sulfate-reducing bacteria in marine corrosive steel

Anaerobic, aerobic, and facultative bacteria are all present in corrosive environments. However, as previous studies to address corrosion in the marine environment have largely focused on anaerobic bacteria, limited attention has been paid to the composition and function of aerobic and facultative bacteria in this process. For analysis in this study, ten samples were collected from rust layers on steel plates that had been immersed in seawater for diff erent periods (i.e., six months and eight years) at Sanya and Xiamen, China. The cultivable aerobic bacterial community structure as well as the number of sulfate-reducing bacteria (SRB) were analyzed in both cases, while the proportion of facultative SRB among the isolated aerobic bacteria in each sample was also evaluated using a novel approach. Bacterial abundance results show that the proportions are related to sea location and immersion time;abundances of culturable aerobic bacteria (CAB) and SRB from Sanya were greater in most corrosion samples than those from Xiamen, and abundances of both bacterial groups were greater in samples immersed for six months than for eight years. A total of 213 isolates were obtained from all samples in terms of CAB community composition, and a phylogenetic analysis revealed that the taxa comprised four phyla and 31 genera. Bacterial species composition is related to marine location;the results show that Firmicutes and Proteobacteria were the dominant phyla, accounting for 98.13% of the total, while Bacillus and Vibrio were the dominant genera, accounting for 53.06% of the total. An additional sixfacultative SRB strains were also screened from the isolates obtained and were found to encompass the genus Vibrio (four strains), Staphylococcus (one strain), and Photobacterium (one strain). It is noteworthy that mentions of Photobacterium species have so far been absent from the literature, both in terms of its membership of the SRB group and its relationship to corrosion.

Effect of Sulfate-reducing Bacteria on Corrosion Behavior of Mild Steel in Sea Mud

Microbiologically influenced corrosion （MIC） is very severe corrosion for constructions buried under sea mud environment. Therefore it is of great importance to carry out the investigation of the corrosion behavior of marine steel in sea mud. In this paper, the effect of sulfate-reducing bacteria （SRB） on corrosion behavior of mild steel in sea mud was studied by weight loss, dual-compartment cell, electronic probe microanalysis （EPMA）, transmission electron microscopy （TEM） combined with energy dispersive X-ray analysis （EDX） and electrochemical impedance spectroscopy （EIS）. The results showed that corrosion rate and galvanic current were influenced by the metabolic activity of SRB. In the environment of sea mud containing SRB, the original corrosion products, ferric （oxyhydr） oxide, transformed to iron sulfide. With the excess of the dissolved H2S, the composition of the protective layer formed of FeS transformed to FeS2 or other non-stoichiometric polysulphide, which changed the state of the former layer and accelerated the corrosion process.

Diversity and community pattern of sulfate-reducing bacteria in piglet gut

Background: Among the gut microbiota,sulfate-reducing bacteria(SRB) is a kind of hydrogen-utilizing functional bacteria that plays an important role in intestinal hydrogen and sulfur metabolism.However,information is lacking regarding diversity and community structure of SRB in the gut of piglets.Middle cecum contents were collected from 6 Yorkshire and 6 Meishan piglets at postnatal days(PND) 14,28 and 49.Piglets were weaned at PND28.Real-time quantitative PCR was performed to detect the number of SRB in the cecum based on dissimilatory sulfite reductase subunit A(dsrA) gene.Prior to real-time PCR,plasmid containing the dsrA gene was constructed and used as external standard to create a standard curve,from which the gene copies of dsrA were calculated.H2S concentration in the cecal contents was measured.Illumina PE250 sequencing of dsrA gene was used to investigate SRB diversity in cecum contents.Results: The qPCR results showed that the number of SRB at PND49 was significantly higher than that at PND28 in Meishan piglets.The concentration of H2S has no significant difference between piglet breeds and between different ages.The Illumina sequencing analysis revealed that the Chao1 richness index was significantly higher at PND49 than that at PND14 and PND28 in Yorkshire piglets.Based on dsrA gene similarities,Proteobacteria,Actinobacteria,and Firmicutes were identified at the phylum level,and most sequences were classified as Proteobacteria.At the genus level,most of sequences were classified as Desulfovibrio.At the species level,Desulfovibrio intestinalis was the predominant SRB in the piglet cecum.The relative abundance and the inferred absolute abundance of Faecalibacterium prausnitzii at PND49 were significantly higher than that at PND14 in Yorkshire piglets.Pig breeds did not affect the dsrA gene copies of SRB,diversity index and community pattern of SRB.Conclusions: Sulfate-reducing bacteria are widely colonized in the cecum of piglets and D.intestinalis is the dominant SRB.The age of piglets,but not the pig breeds affects the diversity and community pattern of SRB.

Influence of Calcareous Deposit on Corrosion Behavior of Q235 Carbon Steel with Sulfate-Reducing Bacteria

Cathodic protection is a very effective method to protect metals, which can form calcareous deposits on metal surface. Research on the interrelationship between fouling organism and calcareous deposits is very important but very limited, especially sulfate-reducing bacteria(SRB). SRB is a kind of very important fouling organism that causes microbial corrosion of metals. A study of the influence of calcareous deposit on corrosion behavior of Q235 carbon steel in SRB-containing culture medium was carried out using electrochemical impedance spectroscopy(EIS), scanning electron microscopy(SEM) and surface spectroscopy(EDS). The calcareous deposit was formed with good crystallinity and smooth surface under the gradient current density of -30 μA cm^(-2) in natural seawater for 72 h. Our results can help elucidate the formation of calcareous deposits and reveal the interrelationship between SRB and calcareous deposits under cathodic protection. The results indicate that the corrosion tendency of carbon steel was obviously affected by Sulfate-reducing Bacteria(SRB) metabolic activity and the calcareous deposit formed on the surface of carbon steel under cathodic protection was favourable to reduce the corrosion rate. Calcareous deposits can promote bacterial adhesion before biofilm formation. The results revealed the interaction between biofouling and calcareous deposits, and the anti-corrosion ability was enhanced by a kind of inorganic and organic composite membranes formed by biofilm and calcareous deposits.

Partial function prediction of sulfate-reducing bacterial community from the rhizospheres of two typical coastal wetland plants in China

Sulfate-reducing bacteria(SRB)are ubiquitous anaerobic microorganisms that play signifi cant roles in the global biogeochemical cycle.Coastal wetlands,one of the major habitats of SRB,exhibit high sulfate-reducing activity and thus play signifi cant roles in organic carbon remineralization,benthic geochemical action,and plant-microbe interactions.Recent studies have provided credible evidence that the functional rather than the taxonomic composition of microbes responds more closely to environmental factors.Therefore,in this study,functional gene prediction based on PacBio single molecular real-time sequencing of 16S rDNA was applied to determine the sulfate-reducing and organic substrate-decomposing activities of SRB in the rhizospheres of two typical coastal wetland plants in North and South China:Zostera japonica and Scirpus mariqueter.To this end,some physicochemical characteristics of the sediments as well as the phylogenetic structure,community composition,diversity,and proportions of several functional genes of the SRB in the two plant rhizospheres were analyzed.The Z.japonic a meadow had a higher dissimilatory sulfate reduction capability than the S.mariqueter-comprising saltmarsh,owing to its larger proportion of SRB in the microbial community,larger proportions of functional genes involved in dissimilatory sulfate reduction,and the stronger ability of the SRB to degrade organic substrates completely.This study confi rmed the feasibility of applying microbial community function prediction in research on the metabolic features of SRB,which will be helpful for gaining new knowledge of the biogeochemical and ecological roles of these bacteria in coastal wetlands.

Corrosion and Electrochemical Behavior of 316L Stainless Steel in Sulfate-reducing and Iron-oxidizing Bacteria Solutions

Corrosion and electrochemical behavior of 316L stainless steel was investigated in the presence of aerobic iron-oxidizing bacteria IOB and anaerobic sulfate-reducing bacteria SRB isolated from cooling water systems in an oil refinery using electrochemical measurement, scanning electron microscopy SEM and energy dispersive atom X-ray analysisEDAX. The results show the corrosion potential and pitting potential of 316L stainless steel decrease distinctly in the presence of bacteria, in comparison with those observed in sterile medium under the same exposure time. SEM morphologies have shown that 316L stainless steel reveals no signs of pitting attack in the sterile medium. However, micrometer-scale corrosion pits were observed on 316L stainless steel sur- face in the presence of bacteria. The presence of SRB leads to higher corrosion rates than IOB. The interactions between the stainless steel surface, abiotic corrosion products, and bacterial cells and their metabolic products in- creased the corrosion damage degree of the passive film and accelerated pitting propagation.

Antimony removal from wastewater by sulfate-reducing bacteria in a bench-scale upflow anaerobic packed-bed reactor

The objective of this research was to study the precipitation of Sb and the stability of Sb2S3 during treatment of synthetic Sb(V)wastewater by sulfate-reducing bacteria(SRB).We investigated the transformation and precipitation of Sb,as well as the re-dissolution of Sb2S3 under different pH and carbon sources.The results showed that the precipitation of aqueous Sb and the re-dissolution of Sb2S3 depend on the conversion between Sb(Ⅲ)-S(-Ⅱ)complexes and Sb2S3 precipitates.These processes were highly pH-dependent,and a decrease in pH enhanced reduction of Sb(Ⅴ)and precipitation of Sb2S3 were observed.The newly formed Sb2S3 was found to be unstable and could re-dissolve through complexation with H2 S due to the increase in pH.When pH decreased to approximately 6.5,Sb was almost completely transformed into Sb2S3 precipitates,and the Sb2S3 precipitates were relatively stable.Compared with lactate as a carbon source,ethanol resulted in a comparable H2 S yield and a relatively low pH and was,therefore,more conducive to the removal of Sb(Ⅴ).The results of this study suggest that when removing Sb(Ⅴ)in wastewater by SRB,it is important to control the pH at a relatively low level.

Synthesis of Chlorinated Bicyclic Adduct as Biocids for Sulfate-Reducing Bacteria

Synthesis of bicyclic systems containing chlorine atoms, and/or ether groups in aromatic rings can be con- sidered as an important method for building bicyclic system and production of new adducts. One of the most important types in the cycloaddition reaction is the Diels-Alder reaction (1,4 cycloaddition). In the present investigation a new ether of allylic type (dienophile) p-allyl bromo phenol was prepared and its structure was confirmed by molecular weight determination, refractive index, infrared spectra, and density. A new adduct was obtained by means of 1,4 cycloaddition reaction of hexachlorocyclopentadiene (HCP) and the new pre- pared dienophile. The reaction takes place without using solvent, catalysts, or elimination of any compound. The effect of variations in temperature, initial molar ratio and reaction duration were studied to determine the optimum conditions of the reaction. The optimum conditions reached were reaction temperature recorded 140?C, initial molar ratio diene: dienophile was 3:1 and the reaction duration time reached 6 h. Under these optimum conditions the maximum yield was 78%. The new adduct revealed very high biological effect as sulfate-reducing bacteria (SRB).

Molecular phylogenetic analysis of sulfate-reducing bacteria from deep sediment layers of the tropical West Pacific warm pool

The diversity of sulfate-reducing bacteria （SRB） from deep layers of deep-sea sediments [ more than 2 m bsf （below seafloor） ] of two sites （WO1 -3 and WPO1 -4） in a tropical West Pacific warm pool region was characterized by using molecular phylogenetic analysis. The results of culture-independent samples demonstrated that the dominant clones from both sites were related to Grampositive spore forming genus, Desulfotomaculum, which accounted for 36.8% of all the sequencing clones from Site WP01 - 3 and 62.8% from Site WP01 -4. However, the other SRB group which was generally reported to be predominant in the deep-sea sediments of other regions, δ- subclass of the proteobacteria was found to be in very low percentages. Therefore, it could be speculated that there existed a unique chemical environment in the deep-sea sediment of this warm pool region. When comparing the Desulfotomaculum sp. related sequences from both sites, it was revealed that though the Desulfotomaculum-like sequences from Site WP01 -3 were more diverse than those from Site WP01 -4, all these sequences from both sites showed high similarity and formed a new phylogenetically homogeneous cluster in the Desulfotomaculum genus which had never been reported before. Successful enrichment of SRB was only achieved from samples of Site WP01 -4 and the sequence analysis of culture-dependent samples further confirmed the dominance of Desulfotomaculum genus. But Desulfotomaculum-related sequences from culture-dependent and culture-independent samples belonged to two different clusters respectively. This difference showed the choice of cultivation to the microorganisms.

Repressing sulfate-reducing bacteria growth in the affusion system of oil field by changing ecological factors

Aiming at the corrosion issue of oil extraction equipments caused by sulfate-reducing bacteria （SRB） reproducing in oil field affusion system, we studied the dominant strains in the SRB community and the impact of four ecological factors on the growth of the dominant strains：temperature, pH, mineralization degree and concentration of PAM （Polyacrylamine）. The feasibility of repressing the growth of SRB by changing ecological factors was also discussed. The results indicate that Desutfobacter （one genus of SRB） is the preponderant strains of the system, and the order of the effect of four ecological factors is pH 〉 temperature 〉 the concentrations of PAM 〉 mineralization degree. The optimal pH for the highest growth rate of SRB is 8.0. No growth of SRB was observed when pH 〈 4 or pH 〉 12. The optimal temperature for the growth of SRB is 40 ℃ and the ecological amplitude is 20 -50 ℃. The appropriate concentration values of PAM is 400 -800 mg/L, beyond of which the multiplication rate and growth quantity 6f cell decrease obviously. The effect of mineralization degree of SO4^2- , HCO^3- and Na^＋ on the growth of SRB has reached an extremely remarkable level, and the change of three ions＇ concentration in water obviously effects SRB： The optimum values on the main ions in the system are Cl- of 200mg/L, HCO^3- of 900 mg/L,SO4^2- of 400 mg/L, Mg^2＋ of 60 mg/L and Na^＋ of 900 mg/L. Our results indicate that it is possible to repress the growth of SRB by changing the ecological factors in nil field affusion system.

Electron transfer from sulfate-reducing bacteria biofilm promoted by reduced graphene sheets

Reduced graphene sheets (RGSs) mediate electron transfer between sulfate-reducing bacteria (SRB) and solid electrodes, and promote the development of microbial fuel cells (MFC). We have investigated RSG-promoted electron transfer between SRB and a glassy carbon (GC) electrode. The RGSs were produced at high yield by a chemical sequence involving graphite oxidation, ultrasonic exfoliation of nanosheets, and N2H4 reduction. Cyclic voltammetric testing showed that the characteristic anodic peaks (around 0.3 V) might arise from the combination of bacterial membrane surface cytochrome c3 and the metabolic products of SRB. After 6 d, another anodic wave gradually increased to a maximum current peak and a third anodic signal became visible at around 0 V. The enhancements of two characteristic anodic peaks suggest that RSGs mediate electron-transfer kinetics between bacteria and the solid electrode. Manipulation of these recently-discovered electron-transport mechanisms will lead to significant advances in MFC engineering.

Se-Bearing Colloidal Particles Produced by Sulfate-Reducing Bacteria and Sulfide-Oxidizing Bacteria: TEM Study

As determined by transmission electron microscopy (TEM), the reduction of selenate and selenite by Desulfovibrio desulfuricans, a sulfate-reducing bacterium, produces spherical (Se, S) sub-micro particles outside the cell. The particles are crystalline or amorphous, depending on medium composition. Amorphous-like Se-rich spherical particles may also occur inside the bacterial cells. The bacteria are more active in the reduction of selenite than selenate. The Desulfovibrio desulfuricans bacterium is able to extract S in the (S, Se) solid solution particles and transform S-rich particles into Se-rich and Se crystals. Photoautotrophs, such as Chromatium spp., are able to oxidize sulfide (S2-). When the bacteria grow in sulfide- and selenide-bearing environments, they produce amorphous-like (S, Se) globules inside the cells. TEM results show that compositional zonation in the (S, Se) globules occur in Chromatium spp. collected from a top sediment layer of a Se-contaminated pond. S2-?may be from the products of sulfate-reducing bacteria. Both the sulfate-reducing bacteria and photosynthetic Chromatium metabolize S preferentially over Se. It is proposed that the S-rich zones are formed during photosynthesis (day) period, and the Se-rich zones are formed during respiration active (night) period. The results indicate that both Desulfovibrio desulfuricans and Chromatium spp. are able to immobilize the oxidized selenium (selenate and/or selenite) in the forms of elemental selenium and (Se, S) solid solutions. The bacteria reduce S in the (Se, S) particles and further enrich Se in the crystalline particles. The reduced S combines with Fe2+ to form amorphous FeS.

Treatment of Agate Dyeing Wastewater Using an Immobilized Gel Mixture with Nano-Fe_(3)O_(4) Sulfate-Reducing Bacteria

To solve the problems of high Cr^(6+),Cr^(3+),SO_(4)^(2-)and H+concentrations,pollution and processing costs associated with agate dyeing industrial drainage,we prepared an immobilized gel mixture for the treatment of such drainage on the basis of microbial immobilization technology.The immobilized gel mixture was composed of sulfate-reducing bacteria(SRB),corn cob,and nano-Fe_(3)O_(4)(nFe_(3)O_(4)).We used a single-factor experiment to determine the optimal dose of each matrix component.We analyzed the mechanism underlying the treatment of agate dyeing wastewater with an immobilized gel mixture by X-ray diffraction and scanning electron microscopy detection.The results of the single-factor test showed that the best treatment was obtained under the following conditions for each matrix component:SRB mass percentage of 30%,nFe_(3)O_(4) dose of 3%,and corn cob mesh size of 100 and dose of 3%.On this basis,we conducted an L9(34)orthogonal experiment to determine the optimal proportion of each matrix component.The results showed that the best treatment was obtained when the gel mixture met the following conditions:SRB mass percentage of 40%,nFe_(3)O_(4) dose of 4%,and corn cob dose of 1%and mesh size of 100.Accordingly,the SO_(4)^(2-),Cr^(6+)and Cr^(3+)removal rates from the agate dyeing drainage were 70.54%,84.75%,and 73.80%,respectively;the total Fe and chemical oxygen demand releases were 1.086 mg·L^(-1)and 1104 mg·L^(-1),respectively;and the pH was 6.27.The gel mixture had the best treatment effect on agate dyeing wastewater under this composition ratio.

Dynamics and Activity of Sulfate-Reducing Bacterial Populations in Paddy Soil under Subsurface Drainage: Case Study of Kamboinse in Burkina Faso

Sulfide toxicity is a common disease generally associated with iron toxicity which occurs in rice fields when the Sulfate-Reducing Bacteria (SRB) produce sulfides ions in anaerobic conditions. The high quantity of sulfides ions in the soil solution upsets the mineral element balance in the rice, affects its growth and causes crop yield losses. In Burkina Faso, many rice field soils are abandoned due to sulfides toxicity. The present study was developed to evaluate the impact of subsurface drainage on SRB dynamics and activity during rice cultivation and the incidence on rice production. Twelve concrete microplots with a clay-loam soil and a rice variety susceptible to sulfides toxicity (FKR 19) were used for the experiment. Soil in microplots was drained for 7 days (P1), 14 days (P2), and 21 days (P3), respectively. Control (T) microplots without drainage were prepared similarly. The evolution of SRB populations and the content of sulfides ions in the paddy soil and in soil near rice roots were monitored throughout the cultural cycle using MPN and colorimetric methods, respectively. Data obtained were analyzed in relation to drainage frequency, rice growth stage, and rice yield using the Student’s t-test and XLSTAT 7.5.2 statistical software. From the results obtained, the subsurface drainage did not affect significantly SRB populations (P = 0.187). However, the drainage affected significantly sulfides concentration in the soil near rice roots (P = 0.032). The concentration of sulfides (P < 0.0001) in soil near rice roots and the number of SRB (P < 0.0001) were significantly higher during the rice tillering and maturity stages. Although no significant difference was observed for rice yield among treatments (P = 0.209), the P2 subsurface drainage showed the highest yield and a low concentration of sulfides in soil near rice roots.

Sulfate-Reducing Bacteria Impact on Copper Corrosion Behavior in Natural Seawater Environment

In this study, the electrochemical corrosion behavior of copper was investigated in seawater collected from four different marine zones of Agadir coastal. These zones are different by the degree of pollution in order to study the effect of this pollution on the copper corrosion, especially the microbial pollution by sulfate reducing-bacteria (SRB). So, to prove this relationship, the microbiological analyses researching the SRB are realized. In parallel, the electrochemical impedance measurement and atomic absorption analysis are established to compare the microbiological evolution cycles with the electrochemical behavior of copper during the immersion period. In the results, we found a good correlation between the growth cycle of marine sulfate-reducing bacteria and the copper corrosion rate by the sulfur and extracellular polymeric substances (EPS) produced as bacteria metabolites. Additionally, this corrosion rate depends on the immersed time: it is maximal after the first or second month depending on the marine zone.

Dynamic coupling of ferrihydrite transformation and associated arsenic desorption/redistribution mediated by sulfate-reducing bacteria

Sulfate-reducing bacteria play an important role in the geochemistry of iron(oxyhydr)oxide and arsenic(As)in natural environments;however,the associated reaction processes are yet to be fully understood.In this study,batch experiments coupled with geochemical,spectroscopic,microscopic,and thermodynamic analyses were conducted to investigate the dynamic coupling of ferrihydrite transformation and the associated As desorption/redistribution mediated by Desulfovibrio vulgaris(D.vulgaris).The results indicated that D.vulgaris could induce ferrihydrite transformation via S^(2-)-driven and direct reduction processes.In the absence of SO_(4)^(2-),D.vulgaris directly reduced ferrihydrite,and As desorption and re-sorption occurred simultaneously during the partial transformation of ferrihydrite to magnetite.The increase in SO_(4)^(2-)loading promoted the S^(2-)-driven reduction of ferrihydrite and accelerated the subsequent mineralogical transformation.In the low and medium SO_(4)^(2-)treatments,ferrihydrite was completely transformed to a mixture of magnetite and mackinawite,which increased the fraction of As in the residual phase and stabilized As.In the high SO_(4)^(2-)treatment,although the replacement of ferrihydrite by only mackinawite also increased the fraction of As in the residual phase,22.1%of the total As was released into the solution due to the poor adsorption affinity of As to mackinawite and the conversion of As^(5+)to As^(3+).The mechanisms of ferrihydrite reduction,mineralogy transformation,and As mobilization and redistribution mediated by sulfate-reducing bacteria are closely related to the surrounding SO_(4)^(2-)loadings.These results advance our understanding of the biogeochemical behavior of Fe,S,and As,and are helpful for the risk assessment and remediation of As contamination.